RestoreSPSite gave no clear indication of what went wrong until I browsed through ulsviewer.exe. There I found: Could not deserialize site from C:\\somefolder\\sharepoint.backups\\wss_content_xyz.bak . Microsoft.SharePoint.SPException: […]

on development environments, it’s better off to set the recovery model to “simple” instead of leaving it on “full”. This had a noticeable improvement. https://blogs.msdn.microsoft.com/jjameson/2011/03/19/using-the-simple-recovery-model-for-sharepoint-development-environments/ […]